Exposure equipment is used to project an image of a circuit pattern from a reticle onto a wafer in semiconductor device production. Resolution, alignment accuracy, and speed are very important for exposure equipment.
The resolution of an exposure tool determines the minimum feature size that can be created on a semiconductor integrated circuit (IC). The minimum feature size is known as the critical dimension (CD). The performance of the IC depends on the number of features that can be packed onto the semiconductor chip, which depends on the CD.
The resolution of an optical projection system is very sensitive to focus. This is why focus finding is important for exposure equipment.
During semiconductor production, a single wafer is exposed with multiple circuit pattern layers. Between exposures of different circuit patterns, the wafer is removed from the exposure tool for processing, such as deposition, etching, or doping with materials that impart semiconducting properties to the wafer. When the wafer returns to the exposure tool, the new layer has to be precisely focused and aligned to the previous layer. This is why alignment accuracy is important for the exposure equipment.
The time spent on high accuracy focus finding and alignment can have a significant impact on the overall speed of the exposure tool. It is also important that focus finding and alignment are robust, to avoid errors and a need for repeat measurements, so that the overall speed of wafer exposure is further increased.
A common method for focus finding is to measure the image contrast as a function of focus position. The best focus is at the position with maximum image contrast. This is known as contrast auto-focus (AF). An example of contrast AF function is shown in FIG. 1, which graphs the relative association between the focus position and the image intensity standard deviation. The standard deviation of the pixel values over an image region is a measure of the image contrast. One problem with contrast AF is that the maximum image contrast may not be known before measurement begins, because it depends on the lens performance and the object contrast. This means that the contrast needs to be measured over a range of focus positions, and then, the lens should be returned to the position with the highest contrast. A second problem with contrast AF is that the contrast function is symmetric for near focus and far focus positions. This means that contrast measurements need to be made for at least two different focus positions, in order to determine whether the lens is at near focus or far focus. A third problem with contrast AF is that the derivative of the contrast with respect to focus position has a minimum value of zero at the best focus position. This means that the method is less sensitive to focus changes when the lens is close to best focus.
U.S. Pat. No. 5,053,799 (Akashi), granted Oct. 1, 1991, and U.S. Pat. No. 5,589,909 (Kusaka), granted Dec. 31, 1996, describe a camera having an auto-focusing device. A portion of the light from the object being imaged is sent to the AF system using a half-silvered mirror. In the AF system, there are two circular sub-apertures located at a conjugate of the main lens aperture. The intensity is measured at a conjugate of the image plane. The intensity peak changes position depending on whether the object is at near focus or far focus. This method is known as phase-detect AF. The phase-detect response is approximately linear and it is asymmetric with near focus and far focus. The advantage of phase-detect AF is that a measurement at a single focus position can be used to determine the distance and direction required to move the lens to best focus. One disadvantage is that the sub-apertures block most of the light from the object, so the signal-to-noise ratio (SNR) may be low. A second disadvantage is that a separate optical path is required for the phase-detect AF system, which makes the camera system more complicated to manufacture, and requires additional calibration.
U.S. Pat. No. 3,013,467 (Minsky), granted Dec. 19, 1961, describes a method of focus finding in a confocal microscope. A pinhole is inserted into the optical path before the illumination lens, in a conjugate plane to the object. A second pinhole is inserted after the collection lens, in the image plane. A detector is placed behind the second pinhole. The intensity of the signal has a maximum at best focus. An advantage of a confocal system is that the best focus can be measured using the intensity rather than the contrast. Disadvantages of a confocal system are that the pinholes must be carefully aligned, the pinholes block a lot of light that reduces the SNR, and the object or the beam must be scanned to build up a two-dimensional (2D) or a three-dimensional (3D) image. Accurate scanning can take a significant amount of time relative to the speed of wafer alignment and exposure in exposure equipment.
U.S. Pat. No. 7,193,685 (Miura), granted Mar. 20, 2007, describes an exposure apparatus that realizes a highly precise focus calibration, using a modified confocal design when the pinholes are replaced with line-space patterns. This modified confocal design has similar advantages and disadvantages to the confocal system.
U.S. Patent Application Publication No. 2008/0137059 (Piestun et. al.), published Jun. 12, 2008, describes a method of estimating the distance between an object and an optical system. The method involves inserting a pupil mask into the aperture of a lens. If the mask is designed using specific Gauss-Laguerre (GL) functions, then the point spread function (PSF) in the image plane has two peaks that rotate as a function of focus position. The PSF rotation angle is approximately linear, and it is asymmetric with near focus and far focus. The rotation angle is detected by deconvolving the PSF. This is called a GL pupil rotating PSF system. An advantage of this system is that a measurement at a single focus position can be used to determine the distance and the direction required to move the lens. This system is very sensitive to focus position, including close to best focus, and operates over a large depth of field. The system can also be used for measurement of three-dimensional object position, such as fluorescent particles in biological microscopy. In biological applications, it is important to be able to make multiple measurements distributed over a large three-dimensional volume. For example, a large number of fluorescent markers may be measured in live biological cell imaging to investigate cell structure and function. Nanometer accuracy is important for analyzing internal features of the cell. At the same time, it is also important to be able to image an entire cell in three dimensions, which may be several micrometers in diameter, due to the motion of features within the cell, and to be able to image a complete functioning live cell without killing the cell by physical dissection.
The GL pupil rotating PSF system could be used for focus finding and alignment in exposure equipment, but there would be disadvantages. One disadvantage is that the system has a low SNR for typical objects with low spatial frequency bandwidth, which means that the accuracy does not meet the minimum required for exposure equipment, and also means that the method would have not have sufficient robustness to variations in wafer processing. A second disadvantage is that the GL pupil design is complicated to design and to manufacture. The complexity of the design and implementation of the GL pupil mask may also cause increased sensitivity to small changes in the exposure tool caused by variations in temperature and pressure, which could lower the accuracy of focus finding in the exposure equipment, or require additional calibration steps. A third disadvantage is that the focus detection method using deconvolution requires capturing two images with different lens aperture diameters, which would increase the exposure tool alignment time. A fourth disadvantage is that the GL pupil design is optimized for measurement over an extremely large depth of field without any change in the PSF, except for scale and rotation. These constraints on the design are not necessary for application in exposure equipment, because only a single surface needs to be aligned, and an initial coarse focus alignment can be performed effectively by other means. These systems all have disadvantages that would reduce the accuracy, robustness, and speed of the focus finding and alignment, and increase the complexity if these methods were used as part of an exposure equipment system.